A torque converter comprises a pump impeller, a turbine runner and a stator supported in a one-way clutch. The pump impeller is connected to the output shaft of an engine through a front cover concentrically affixed thereto. The turbine runner is connected to the input shaft of a transmission gear.
In an automatic transmission for a motor vehicle, a direct-coupling clutch is provided within the torque converter for directly connecting the output shaft of the engine and the input shaft of the transmission gear. For this purpose, a clutch disk is mounted axially slidably on the input shaft of the transmission gear and is adapted to be rotatable together with the turbine runner in the space formed between the front cover and the turbine runner, while a flat surface is formed on the front cover for affixing a frictional material or for frictionally engaging with a friction material affixed to the clutch disk. The output shaft of the engine and the input shaft of the transmission gear are directly connected by the frictional engagement of the clutch disk and the front cover through the frictional material.
It is necessary that a friction surface formed on the front cover for affixing a frictional material or for engaging with a frictional material is quite flat in order to provide the direct-coupling clutch with an increased capacity of torque transmission and to prevent the irregular wear of the frictional material.
However, since the front cover of the torque converter is manufactured by press-forming a metal plate in order to attain a high productivity as well as low manufacturing cost, the friction surface is subject to deflection and corrugation due to the spring-back phenomenon of the press-formed plate. Besides, the front plate is liable to be partially deformed during the welding of mounting seats and a center guide to the front cover.
In order to remove the irregularity originating in such deflection, corrugation and/or partial deformation, the friction surface is finished by grinding or cutting the irregularity by means of a grinding machine or a cutting machine as deep as to a surface lying at a predetermined distance from a zero-surface. Such a finishing process sometimes results in an excessively thin thickness of the front cover, which is liable to cause cracking of the front cover.